The present invention relates generally to gas turbine engines, and, more specifically, to turbine blade cooling therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases which flow downstream through turbine stages that extract energy therefrom. A high pressure turbine powers the compressor, and a low pressure turbine powers an upstream fan in a turbofan aircraft engine embodiment.
The first stage turbine blades first receive the hot combustion gases from the combustor and are typically air cooled by using air bled from the compressor. Turbine blade cooling is quite esoteric and the art is well crowded in view of the complex nature of blade cooling.
A typical turbine blade includes a generally concave pressure side and an opposite, generally convex suction side extending axially or chordally between leading and trailing edges which extend radially in span from root to tip of the blade. The airfoil portion of the blade is hollow and extends radially outwardly from a supporting dovetail which mounts the blade in a supporting rotor disk.
Cooling air is channeled to each blade through the dovetail and various internal passages are formed inside the airfoil for tailoring cooling thereof to mitigate the various heat loads experienced around the outer surface of the airfoil.
The radially outer end or tip of the airfoil is particularly difficult to cool since it is exposed to hot combustion gases along both the pressure and suction sides of the airfoil as well as in the radial clearance or gap formed with the surrounding stator casing or shroud. Since turbine blades are subject to occasional tip rubs, the airfoil tip is typically formed by squealer rib extensions of the pressure and suction sides which join together at the leading and trailing edges and define an open tip plenum therebetween having a floor which encloses the internal passages of the airfoil.
A significant advancement in blade tip cooling is U.S. Pat. No. 5,261,789 which discloses the use of a tip shelf along the pressure side of the turbine blade. The tip shelf is fed with cooling air through holes formed therethrough and interrupts the flow of combustion gases along the pressure side of the blade tip. Improved cooling of the blade tip including the pressure side tip rib is obtained.
During operation, combustion gases flow axially over the pressure and suction sides of the airfoil, with a portion thereof migrating radially upwardly along the pressure side and over the pressure side tip rib where it leaks past the airfoil tip in the small gap formed with the shroud. The resulting flow field of the combustion gases and cooling air discharged from the tip shelf is complex and affects both aerodynamic performance of the airfoil and cooling of the tip ribs themselves which are solid members extending upwardly from the tip floor.
Although the tip shelf and ribs are relatively small features of the airfoil, the importance thereof cannot be overstated since oxidation of the tip and material lost therefrom limits the useful life of the blade. The tip ribs are typically manufactured by casting with the entirety of the blade itself, and the small tip shelf may also be formed by casting or by electrical discharge machining (EDM) where required or practical. In either manufacturing method, the pressure side tip rib and cooperating tip shelf have dimensions measured in several mils, and are thus subject to manufacturing tolerances which affect the performance thereof.
Furthermore, the individual tip ribs are subject to centrifugal loading during operation which generates corresponding stress at the bases thereof with the tip floor. And, the tip shelf joins the pressure side tip rib at a correspondingly small fillet at which centrifugal stress may be concentrated during rotation of the blades in operation.
Accordingly, it is desired to provide a turbine blade having improved tip cooling notwithstanding manufacturing tolerances and centrifugal loads.
A gas turbine engine blade includes pressure and suction sidewalls extending between leading and trailing edges and from root to tip. The pressure sidewall includes an inclined tip rib offset therein by a ramp defining a tip notch having compound inclinations.